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Searching for Solar Axions Using Data from the Sudbury Neutrino Observatory

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Searching for Solar Axions Using Data from the Sudbury Neutrino Observatory PHYSICAL REVIEW LETTERS 126, 091601 (2021) Searching for Solar Axions Using Data from the Sudbury Neutrino Observatory Aagaman Bhusal ,1,2 Nick Houston ,3,* and Tianjun Li1,2 1CAS Key Laboratory of Theoretical Physics, Institute of Theoretical Physics, Chinese Academy of Sciences, Beijing 100190, China 2School of Physical Sciences, University of Chinese Academy of Sciences, No. 19A Yuquan Road, Beijing 100049, China 3Institute of Theoretical Physics, Faculty of Science, Beijing University of Technology, Beijing 100124, China (Received 20 April 2020; accepted 1 February 2021; published 2 March 2021) We explore a novel detection possibility for solar axions, which relies only on their couplings to nucleons, via the axion-induced dissociation of deuterons into their constituent neutrons and protons. An opportune target for this process is the now-concluded Sudbury Neutrino Observatory (SNO) experiment, which relied upon large quantities of heavy water to resolve the solar neutrino problem. From the full SNO 3 ≡ 1 − dataset we exclude in a model-independent fashion isovector axion-nucleon couplings jgaNj 2 jgan −5 −1 gapj > 2 × 10 GeV at 95% C.L. for sub-MeV axion masses, covering previously unexplored regions of the axion parameter space. In the absence of a precise cancellation between gan and gap this result also exceeds comparable constraints from other laboratory experiments, and excludes regions of the parameter space for which astrophysical constraints from SN1987A and neutron star cooling are inapplicable due to axion trapping. DOI: 10.1103/PhysRevLett.126.091601 Introduction.—Arising straightforwardly as a minimal experimental convenience, most of the existing axion extension of the standard model, and in particular the literature rests similarly on axion-photon interactions. Peccei-Quinn solution of the strong CP problem [1–3], We will in the following instead turn attention to a less axions and axionlike particles (ALPs) occupy a rare focal well-explored corner of the parameter space, and, in point in theoretical physics, in that they are also simulta- particular, the axion-nucleon interactions neously a generic prediction of the exotic physics of string and M theory compactifications [4,5]. Despite the profound 1 μ 1 μ L ¼ g ð∂μaÞn¯γ γ5n þ g ð∂μaÞp¯ γ γ5p; ð1Þ differences between these contexts the resulting axion 2 an 2 ap properties are largely universal, creating an easily charac- terizable theoretical target. which can be reexpressed in terms of the neutron-proton As typically light, long-lived pseudoscalar particles they doublet N ¼ðn; pÞ as can also influence many aspects of cosmology and astro- 1 physics, leading to a wealth of observational signatures [6]. ¯ μ 1 3 L ¼ ð∂μaÞNγ γ5ðg I þ g τ3ÞN; ð2Þ In particular, they provide a natural candidate for the 2 aN aN mysterious dark matter comprising much of the mass of our visible universe [7,8], and as such are a focal point of where the isoscalar and isovector couplings are, respec- 1 2 3 − 2 intense ongoing investigation [9]. tively, gaN ¼ðgan þ gapÞ= , gAN ¼ðgan gapÞ= , and Fortuitously, our own Sun should provide an intense and τ3 ¼ diagð1; −1Þ. Although we will not make use of this readily available axion flux from which much of the here, if required integration by parts can be used to recast corresponding parameter space can be constrained. At this into present, the strongest resulting constraint is provided by L − ¯ γ 1 3 τ the CAST experiment [10], which relies upon the Primakoff ¼ iaN 5mNðgaNI þ gaN 3ÞN; ð3Þ conversion of axions into photons in a background mag- netic field. Presumably for reasons of observational and where mN ¼ diagðmn;mpÞ. We also emphasise here that although we consider axion-nucleon interactions, we are not assuming a QCD axion specifically. There are comparatively few constraints on axion- nucleon interactions, arising primarily from considerations Published by the American Physical Society under the terms of of SN1987A [11–14] and neutron star (NS) observations the Creative Commons Attribution 4.0 International license. – Further distribution of this work must maintain attribution to [15 18], experimental searches for new spin-dependent the author(s) and the published article’s title, journal citation, forces [19–25] and time dependent nuclear electric dipole and DOI. Funded by SCOAP3. moments [26]. It should, however, be noted that the overall 0031-9007=21=126(9)=091601(7) 091601-1 Published by the American Physical Society PHYSICAL REVIEW LETTERS 126, 091601 (2021) paradigm of axion constraints derived from SN1987A has The constant of proportionality is the probability for a been called into question [27,28]. given M1 nuclear transition to result in axion rather than The CASPER experimental program is also searching photon emission, for axion dark matter via nuclear interactions [29–31], and Γ 1 2 β 1 3 2 3 similar considerations have led recently to novel constraints a ≃ mn gaN þ gaN pa 2 ; ð6Þ from existing comagnetometer data [32]. Γγ 2πα1 þ δ ðμ0 − 0.5Þβ þ μ3 − η pγ Limits also exist from rare meson decays [33] and dedicated solar axion experiments which rely only where α is the fine structure constant, δ2 ¼ E=M is the upon nuclear couplings [34–39],however,astheseare relative probability for E and M transitions, we set model dependent we will not consider them going mn ¼ mp, and μ0 ¼ μp þ μn ≃ 0.88 and μ3 ¼ μp − μn ≃ forward. 4.71 are, respectively, the isoscalar and isovector nuclear As we will demonstrate, there is also an entirely novel magnetic moments, and pa=γ are the axion-photon and model-independent constraint arising from axions momenta [41]. Dependence on specific nuclear matrix emitted during nuclear transitions inside the Sun, which elements enters through β and η. can be detected on Earth through the M1 process In Eq. (5), the M1-type transitions associated with axion emission correspond to capture of protons with zero orbital a þ d → n þ p; ð4Þ momentum. The probability of this occurring at a proton energy of 1 keV has been measured and found to be 0.55 [42], implying δ2 0.82. Since capture from the S state where d is a deuteron. A particularly opportune target for ¼ corresponds to an isovector transition, we can assume the this mechanism is the now-concluded Sudbury Neutrino βg1 contribution to Eq. (6) is negligible and, to a good Observatory (SNO) experiment, which relied upon a large aN approximation, also ignore everything other than μ3 in the quantity of deuterium to resolve the solar neutrino problem denominator [43], leading to [40]. The possibility of using deuterium for axion detection was first noted by Weinberg in Ref. [2]. Γ 1 m2 g3 2 p 3 After detailing the corresponding axion flux and inter- a ≃ n aN a : Γ 2πα1 δ2 μ ð7Þ action cross section for Eq. (4) next, we then compute the γ þ 3 pγ resulting axion-induced event rate in SNO and derive constraints therefrom. Conclusions and discussion are Following Ref. [44] this provides the flux at Earth due to presented in closing. Eq. (5), Solar axion flux.—While axions can in general be ϕ ≃ 3 23 1010 2 3 2 3 −2 −1 produced within stars by a number of processes, such as a . × mnðgaNÞ ðpa=pγÞ cm s : ð8Þ Primakoff conversion, electron bremsstrahlung, and Compton scattering, we are primarily interested here in As this component of the solar axion flux has already their production via low-lying nuclear transitions. This is been well explored by the Borexino and CAST experiments primarily because the threshold for deuterium dissociation via a number of detection channels [44,45], we will in the via Eq. (4) is 2.2 MeV, and so only solar axions arising following focus only on the previously unexplored detec- from nuclear transitions will have sufficient energy. tion channel provided by deuterium (4). — However, there is also secondary benefit in so doing Axiodissociation cross section. We now construct the “ ” in that both emission and detection will rely only upon cross section for the axiodissociation process a single axion-nucleon coupling, allowing a model- → independent constraint of general applicability. a þ d n þ p; ð9Þ Of the nuclear transitions occurring inside the sun, the most intense axion flux is provided by with threshold energy 2.2 MeV. For reasons of clarity and brevity we give a relatively simple derivation of this quantity, postponing more thorough study to the future. → 3 γ 5 5 p þ d He þ ð . MeVÞ; ð5Þ Details of the corresponding nuclear physics are found in Refs. [46,47]. where an axion substitutes for the emitted gamma ray. In Working in the rest frame of the initial state deuteron, the the standard solar model (SSM) this constitutes the second number of events we expect is stage of the pp solar fusion chain, with the first stage → þ ν provided by the two reactions p þ p d þ e þ e and Ne ¼ σϕNdT; ð10Þ − p þ p þ e → d þ νe. As the deuterons produced via this first stage capture protons within τ ≃ 6 s, the axion flux where ϕ is the incoming flux, Nd the number of targets and resulting from Eq. (5) can be expressed in terms of the T the time, we can rearrange for a single deuteron to give known pp neutrino flux. σ ¼ðV=viÞðNe=TÞ, where we have used the fact that for a 091601-2 PHYSICAL REVIEW LETTERS 126, 091601 (2021) single incoming axion ϕ ≡ nivi ¼ vi=V. No integration where m ≃ mn=2 is the reduced mass, r is the distance over energy is required since thermal broadening is between nucleons, ED ¼ 2.2 MeV the binding energy of negligible relative to the axion energy at hand.
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